After almost two years ailing behind bars in the bowels of an Iranian prison and coming down with the novel coronavirus, cancer survivor Michael White last week was freed and brought home.

The U.S. Navy veteran joins the ranks of scores of Americans brought home under President Trump's aggressive push to free unfairly detained citizens – yet there are many others still languishing in all corners of the globe.

"Iran is a prime example of the different approaches taken by the current and previous U.S. administrations. While diplomacy was an essential feature of both, in each instance, it was wielded differently," Behnam Ben Taleblu, a senior fellow and Iran expert at the Foundation for Defense of Democracies (FDD), told Fox News. "Specifically, the Trump administration has been able to maintain a policy of maximum pressure on Iran while securing the release of Americans. This was not the case in 2016, where a fatally flawed nuclear deal went into effect, and pallets of cash were delivered."

While there is believed to be thousands of U.S. citizens incarcerated abroad, the U.S. State Department distinguishes those alleged to have been involved in crimes with those considered to be political pawns, sometimes referred to as hostages, held by foreign governments and often aimed at leveraging deals or purporting to coerce the U.S. government.

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The number of exact political prisoners remains classified, although one former government officials closely connected to the matter told Fox News that the number is likely between 50 and 100 and spawning multiple countries. Not all cases are made public, for varying reasons in that often diplomats prefer to handle negotiations quietly, but many are brought into the limelight often by frustrated family members hoping to garner media attention.

So what Americans are known to still be detained abroad?

(The U.S. is still seeking freedom for businessmen Baquer Namazi; and his son Siamak Namazi, who remain in Iran under dubious charges. (family provided))

IRAN

The U.S. is still seeking freedom for businessmen Baquer Namazi and his son Siamak Namazi, who remain in Iran under dubious charges.

Baquer, a dual Iranian-American citizen, was apprehended by authorities while on business in Iran – along with his son –  in 2015 and both were sentenced to 10 years in prison for "collusion with an enemy state," pointing to the United States.

While Siamak remains in the notorious Evin Prison in Tehran, 84-year-old Baquer "is out of prison but still very sick and not allowed to leave Iran despite his frail condition and need for urgent medical attention," his son Babak Namazi said in a statement last week.

Meanwhile, Morad Tahbaz – a fellow Iranian-American who also holds British citizenship – was arrested in early 2018 and last year slapped with 10 years imprisonment for "contact with the enemy U.S. government." Tahbaz is a well-known conservationist and co-founder of the Persian Wildlife Heritage Foundation, which aims to study and protect the endangered Asiatic cheetah.

In a statement regarding White's release last week, Secretary of State Mike Pompeo additionally called for their release, stressing that "the United States will not rest until we bring every American detained in Iran and around the world back home to their loved ones."

RUSSIA

The U.S. is yet to bring home Paul Whelan from Russia. The former U.S. Marine, 50, was arrested by security agents in Moscow in December 2018, accused of having a USB drive that contained classified information and of being a foreign agent.

Paul Whelan, a former U.S. Marine, center, who was arrested in Moscow at the end of last year, waits for a hearing in a court in Moscow, Russia, Friday, May 24, 2019. The American was detained at the end of December for alleged spying. (AP Photo/Pavel Golovkin)

At the time, Whelan – who had traveled to Russia multiple times since 2006 – said he returned to attend a wedding when he was seized by authorities. Prosecutors are seeking an 18-year prison sentence for Whelan, who holds multiple passports, including the U.K. and Ireland. Concerns have also been sparked for his health, given that he underwent emergency hernia surgery late last month.

His trial is ongoing.

SYRIA

In Syria, the government of Bashar al-Assad's government is believed to have detained at least two Americans: psychologist Majd Kamalmaz and freelance journalist Austin Tice.

Kamalmaz, according to the FBI, is a Syrian-American who was treating refugees in the region from war-ravaged Syria. In February of 2017, the now 62-year-old traveled to his native land to visit an elderly family member in Damascus.

"During this trip, he was also looking to establish a clinic to aid those who have been traumatized by the Syrian civil war. A day after arriving, Kamalmaz was stopped at a Syrian Government checkpoint in Mezzeh, a suburb of Damascus, and has not been seen or heard from since that day," U.S. officials stated. "Kamalmaz is diabetic and requires regular medication."

Freelance journalist Austin Tice, pictured here in an undated photo, was kidnapped in Syria in August 2012.

Freelance journalist Austin Tice, pictured here in an undated photo, was kidnapped in Syria in August 2012. (Handout)

And there is former U.S. Marine-turned-journalist Austin Tice, 37, who vanished amid the nation's brutal civil war in 2012. After his disappearance, a video emerged of him blindfolded and being marched into the hills at the behest of what appears to be armed insurgents; however, multiple officials have since claimed that such a clip was likely a ruse and that he was jailed by the Syrian regime.

In December 2018, Tice's parents announced during a press conference that they had received new proof-of-life information indicating that he was still alive.

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VENEZUELA

As relations between the U.S. and Venezuela have continued to sour, concerns were heightened earlier this year with regards to the wellbeing of the "CITGO 6" – five U.S. citizens and one legal resident who have been detained by the Maduro regime for more than two-and-a-half years.

Alirio Zambrano and his brother Luis Zambrano, and former CITGO vice presidents Jorge Toledo, Tomeu Vadell, and Gustavo Cardenas and president Jose Pereira remain in custody.

"Still to this day, he has not been allowed to see his lawyer. No one has really seen him; we don't know what he looks like or what his condition is," Veronica Vadell, the daughter of Vadell, told Fox News on Monday. "It has been 930 days. Now with COVID and everything there on lockdown, all we can do is sit around in limbo."

Fears for the health and safety of Tomeu Vadell, who is among the CITGO 6 detained in Venezuela (family handout)

Veronica noted that while they receive "sporadic phone calls" from their father, who can say very little, he is technically in a state of "forced disappearance" as he has not been able to visit with a lawyer and nobody has confirmed his presence in the maximum security facility.

"My father needs to be released on a humanitarian basis. He is going to be 61 in August, and he has cardiovascular and respiratory issues," she continued. "We're desperate for anything to happen."

His trial was slated to begin in March but was canceled as the global cornavirus pandemic struck, and a new date is yet to be set.

The American oil executives employed by the Houston-headquartered CITGO, a division of Venezuelan state oil company PDVSA, were initially seized by officials in late November 2017 – after being lured to the Venezuelan capital of Caracas for a meeting – and slapped with corruption charges.

More recently, former U.S. Special Forces operators Airan Berry and Luke Denman were detained in early May following a bizarre, ill-equipped effort to overthrow Maduro. The Trump administration has rejected any involvement in the operation, which was foiled by agents loyal to the Caracas leadership long before it was executed.

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CHINA

Americans who operated an English teaching program, China Horizons, remain under the thumb of Chinese authorities.

On Sept. 27, 2019, Idaho native Alyssa Petersen, 28, the director of the program – which enabled college students to visit China and teach English as a second language –  was suddenly arrested near Shanghai. Two days later, the company's owner and founder, Jacob Harlan, was also detained by police under the guise that they were both "illegally moving people across borders."

Last November, Petersen, and Harlan were released on bail, but the family has said the two have since been taken back into custody and that the prison is under lockdown as a result of the novel coronavirus.

According to updates issued by Peterson's family on a GoFundMe page seeking financial aid to bring her home, her case has reached the "prosecution phase."

"She wakes up when told; she goes to sleep when told. She spends her day in a Jail Cell or walking in a circle counting steps.  She cannot have any contact with anyone outside of a Consulate Officer who can visit once a month and a Lawyer," Peterson's sister wrote. "The investigation could take months, even years without proper legal representation. Alyssa needs a Lawyer Now, and they aren't cheap. We are desperately seeking help to raise funds to offset the substantial legal fees and fines  that will occur as we struggle to help Alyssa gain her freedom."

John Cao, 60, a North Carolina-based pastor and lawful resident, is also being held on the same charge of illegal border maneuvering.

Cao – who turned to Christianity after coming to America in his 20s– devoted much of his life to building schools and providing financial aid to ethnic minorities oppressed in Burma, but was sentenced to seven years behind bars in China in March 2018 in what many officials deem a "wrongful conviction."

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According to Michael Rubin, resident scholar at the American Enterprise Institute (AEI), the number of Americans detained under dubious circumstances fluctuates with time.

"The three biggest offenders in recent years have been Iran, North Korea, and Cuba, with Lebanon and Egypt also holding Americans prisoner from time to time," he said. "The problem in many of these countries – Iran, Russia, Cuba, etc., is they tend not to recognize American citizenship for those who had emigrated to America. It takes an act of parliament, for example, for Iran to recognize the revocation of Iranian citizenship even if an Iranian American doesn't want to be considered Iranian. Too many dual national travelers make it worse by – for the sake of ease – traveling on a non-American passport."

The U.S. State Department did not immediately respond to a request for comment.

On its website, it vows that one of its highest priorities is aiding Americans who have been incarcerated. However, the department stresses that it can ensure prison officials are providing appropriate medical care, contact family and friends and provide a list of English-speaking attorneys, it cannot merely get citizens out of jail, pay legal or medical fees, or state to the court whether someone is innocent or guilty.

Following failed efforts to free U.S. journalist James Foley from ISIS captivity in Syria, the Obama administration in 2014 developed a "hostage fusion cell" to better coordinate the release process and ensure that the government, intelligence agencies, and the FBI were adequately sharing information and that coordinated responses were in place.

"There should be no rank ordering of U.S. hostages abroad. Wherever Americans are detained abroad, priority number one must be their safe and speedy return," added Taleblu. "No jurisdiction or person matters more than any other."